Photovoltaic solar cell fabrication often involves the process of “co-firing” where metallic electrical conductors are mechanically and electrically coupled to semiconductor materials to create an electrical p-n semiconductor photovoltaic device. For example, during fabrication, an aluminum (Al) “ink” would be applied to a back surface (e.g., a p-type region) of a wafer of semiconductor material (such as a Silicon (Si) wafer). The opposing front surface (e.g. an n-type region layer) of the wafer would have a coating of silicon nitride (SiN) applied and a pattern of silver (Ag) “ink” applied over the SiN. The device is then fired (that is, heated in a furnace) such that the Ag ink dissolves through portions of the SiN layer to make electrical contact with the n-type region and the Al and Si at the back surface dissolve and then re-grow to form a good electrical and mechanical connection.
One problem associated with the fabrication of a photovoltaic solar cell involves the purity level of a semiconductor (e.g. Si) material. The purity level is often a function of the production facility used. That is, production of semiconductors wafers with little to no impurities usually requires tightly controlled clean-room environments. For industrial facilities attempting to produce Si wafers on a mass-production scale for photovoltaic solar cell devices, maintaining strict clean-room environments is economically prohibitive. Therefore, some degree of impurities can be expected to exist in the final fabricated wafers. Such industrial facilities may therefore employ one or more post-processing techniques to attempt to rid impurities from the wafers. For example, if Si layers of a photovoltaic device include dissolved (that is, non-precipitated) impurities within the silicon material, then optical processing can be used to cleanse the wafer. Under optical processing, a diffusion process (called gettering) can be initiated that will cause the dissolved impurities to migrate into the Aluminum contact material. For example, by initiating diffusion, dissolved iron (Fe) impurities can rapidly (i.e., in the matter of a few minutes) be removed from the Si layers. This serves to clean impurities from the silicon wafer. A problem occurs however, when the impurities include precipitated material. In that case, diffusion will not cause movement of the precipitated impurities from the silicon layers. Further, the durations and high temperatures required to dissolve the precipitate impurities may not be compatible with other co-firing process steps, and have the potential to increase manufacturing expenses.
In accordance with common practice, the various described features are not drawn to scale but are drawn to emphasize relevant features. Reference characters denote like elements throughout Figures and text.